Convenient methods for preparing ?-conjugated linkers as building blocks for modular chemistry

Article abstract of DOI:10.3762/bjoc.5.11

Simple, straightforward and optimized procedures for preparing extended ?-conjugated linkers are described. Either unsubstituted or 4-donor substituted ?-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl ?-conjugated backbone are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building blocks in Suzuki-Miyaura or Sonogashira coupling reactions.

Full text of DOI:10.3762/bjoc.5.11

Convenient methods for preparing π-conjugated
linkers as building blocks for modular chemistry
Jiří Kulhánek, Filip Bureš* and Miroslav Ludwig
Full Research Paper
Open Access
Address:
Beilstein Journal of Organic Chemistry 2009, 5, No. 11.
Institute of Organic Chemistry and Technology, Faculty of Chemical
Technology, University of Pardubice, nám Čs. legií 565, Pardubice,
532 10, Czech Republic
doi:10.3762/bjoc.5.11
Received: 12 February 2009
Accepted: 07 April 2009
Published: 14 April 2009
Email:
Filip Bureš* - filip.bures@upce.cz
Associate Editor: P. Skabara
* Corresponding author
© 2009 Kulhánek et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
boronic acid; donor/acceptor; linker; Sonogashira reaction; property
tuning; push-pull; Suzuki–Miyaura reaction
Abstract
Simple, straightforward and optimized procedures for preparing extended π-conjugated linkers are described. Either unsubstituted
or 4-donor substituted π-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl π-conjugated backbone
are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building
blocks in Suzuki–Miyaura or Sonogashira coupling reactions.
Introduction
Development of new organic compounds with improved and matic rings as well as their combinations [14-19]. A typical
advanced properties is one of the most important goals of synthetic approach to CT chromophores involves either a step-
modern material chemistry. Organic chemists steadily attempt wise formation of the target molecule [19,20] or a separate
to design and synthesize novel and well-defined organic push- preparation of the donor as well as the acceptor moieties and
pull systems with prospective applications as chromophores for their final combination [21,22]. It is already well known that the
nonlinear optics (NLO) [1-5], dyes [6], electronic and photonic HOMO/LUMO gap and polarizability of the push-pull systems
devices [7,8], organic light-emitting diodes (OLED) [9] or func- with the given donors and acceptors can be finely tailored by
tional polymers [10-13]. A typical push-pull chromophore the extension or shortening of the π-conjugated path between
consists of a polar A-π-D system with a planar π-system end- the donor and acceptor [19,21,23-25]. Thus, the latter modular
capped by a strong electron donor (D) and a strong electron synthetic approach seems to be more suitable for the property
acceptor (A). The π-conjugated system ensuring charge-transfer tuning described above. The final combination, C–C bond form-
(CT) between the donor (D = NR2, OR groups etc.) and the ation, of the donor and acceptor chromophore moieties is
acceptor (A = NO2, CN groups etc.) is most commonly usually accomplished by cross-coupling reactions, in particular
comprised of double and triple bonds, aromatic and heteroaro- by the Suzuki–Miyaura [26,27] or the Sonogashira [28] reac-
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Beilstein Journal of Organic Chemistry 2009, 5, No. 11.
tions. Consequently, the availability of the suitably substituted by Perner and co-workers [33] proved to be infeasible.
π-conjugated linkers of various lengths bearing boronic ester However, in light of the report by Itami and Yoshida [34], we
functionality or terminal acetylene is crucial for such a synthetic attempted the Mizoroki–Heck C–H arylation of 4-bromo-N,N-
approach. Hence, we report here a convenient synthesis as well dimethylaniline with an equimolar amount of vinylboronate
as characterization of either unsubstituted (R = H) or donor pinacol ester leading to the desired 3c in 73% yield (Scheme 1,
substituted (R = NMe2, OMe) π-conjugated linkers designed for Method A).
the Suzuki–Miyaura and Sonogashira cross-couplings with a
systematically varied and enlarged π-conjugated path (Figure 4-Substituted 4′-bromobiphenyl intermediates necessary for the
1).
preparation of 4a–c were synthesized by the Suzuki–Miyaura
cross-coupling of 1-bromo-4-iodobenzene with the corres-
Whereas the simplest linkers such as 4-substituted phenyl- ponding boronic acids/esters 1a–c in the yields of 82, 84, and
boronic pinacol esters 1a–c [29] and ethynylbenzenes 2a–c [30]
are well known and also commercially available, the proposed
Table 1: Optimized synthetic procedures and yields for the prepara-
dioxaborolanes 3–6 feature styryl (series 3), biphenylyl (series
4), (E)-phenylethenylphenyl (series 5), and phenylethynyl-
phenyl (series 6) linkers and H (series a), OMe (series b), and
NMe2 (series c) groups as the substituent R, respectively
(Figure 1, Table 1). The terminal acetylenes 7c–9c possess only
the strongest NMe2 donor and have an identical backbone to the
one described above (Figure 1, Table 1).
tion of 3–9.
Entry
Product (R)
Method
Yield (%)
1
3c (NMe2)
4b (OMe)
4c (NMe2)
5a (H)
A
B
B
C
C
C
D
D
D
E
E
E
73a
2
81a
3
83a
4
76b
5
5b (OMe)
5c (NMe2)
6a (H)
69a
6
82a
Results and Discussion
Synthesis of boronic pinacol esters 3–6
7
78a
8
6b (OMe)
6c (NMe2)
10/7c
72a
Whilst two styryl dioxaborolanes 3a–b are known [31,32] and
commercially available, the N,N-dimethylamino substituted
derivative 3c needed to be synthesized. In order to achieve pure
(E)-3c, at first, a hydroboration of the commercially available
terminal acetylene 2c with catecholborane was examined.
Despite all attempts to optimize the reaction conditions, 3c
could not be prepared this way and was not even detected in the
crude reaction mixture. Thus the above hydroboration reported
9
91a
10
11
12
98/92c
97/91c
99/89c
11/8c
12/9c
aYield of the final coupling step. bHorner–Wadsworth–Emmons reaction.
cYield of the Sonogashira cross-coupling (10–12) and final TMS-group
removal to the terminal acetylenes 7c–9c.
Figure 1: Basic and newly proposed π-conjugated linkers designed for the Suzuki–Miyaura and Sonogashira cross-coupling.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 11.
Scheme 1: Convenient synthetic methods leading to π-linkers 3–6.
91%, respectively. A routine procedure involving a lithiation could be most effectively converted to target pinacol esters
and reaction with triisopropyl borate followed by esterification 5a–c via borylation (Scheme 1, Method C) utilizing
with pinacol afforded dioxaborolanes 4b–c in the yields of 81 bis(pinacolato)diboron (pin2B2) [39] in a mixed solvent system
and 83% (Scheme 1, Method B). The biphenyl-4-boronic acid DMSO/dioxane ensuring good solubility. It should be noted
corresponding to 4a was also commercially available while 4b here that 5a–c were also accessible via the routine sequence
was reported as a side product [35] without full characteriza- showed for Method B. On the contrary, borylation of
tion.
4-bromobiphenyls with pin2B2 (Method C) yielded only traces
of 4a–c.
Whereas 5a was easily accessible as a pure (E)-product from
the pinacol ester of 4-formylphenylboronic acid and diethyl Linear phenylethynylphenyl π-linkers 6a–c were gained by a
benzylphosphonate through the Horner–Wadsworth–Emmons Sonogashira cross-coupling between the pinacol ester of
reaction in 76% yield [36], methoxy and N,N-dimethylamino 4-iodophenylboronic acid [39] and terminal acetylenes 2a–c
substituted (E)-4-bromostilbenes were synthesized from the (Scheme 1, Method D) in 72–91% yield. Table 1 (entries 1–9)
corresponding benzaldehydes and 4-bromobenzyl(triphenyl)- summarizes the used synthetic methods and yields for the
phosphonium bromide [37] by the Wittig reaction [37,38] in 37 particular dioxaborolanes 3–6.
and 54% yields, respectively. In contrast to the Horner–Wads-
worth–Emmons reaction, this procedure afforded both (E)- and Synthesis of terminal acetylenes 7–9
(Z)-stilbenes that were isomerized by heating with traces of Synthesis of N,N-dimethylamino substituted terminal acet-
iodine in toluene to afford pure (E)-stilbenes [38]. However, ylenes 7c–9c was accomplished by Sonogashira cross-coupling
compared to N,N-dimethylaminostilbene, the isomerization of as shown on the Scheme 2 (Method E). The reaction utilizes the
4-bromo-4′-methoxystilbene required twelve times the 4-bromo derivatives used as precursors for the construction of
prolonged reaction time (4 vs. 48 h) while the isomerisation of dioxaborolanes 4–6 and the product of the Sonogashira cross-
unsubstituted 4-bromostilbene did not take a place at all. Hence, coupling between 2c and 1,4-diiodobenzene (54% yield). Since
5a had to be prepared by the Horner–Wadsworth–Emmons the Sonogashira reaction between bromo derivatives and
reaction described above. These substituted 4-bromostilbenes trimethylsilylacetylene proved to be sluggish and low yielding
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Beilstein Journal of Organic Chemistry 2009, 5, No. 11.
Scheme 2: Sonogashira cross-coupling leading to π-linkers 7c–9c.
(even with a large excess of acetylene and elevated
temperature), the bromo derivatives were converted to the
corresponding iodo derivatives by lithiation and quenched with
iodine (see Supporting Information File 1). Thus the Sono-
gashira reaction on iodo derivatives smoothly furnished TMS-
protected acetylenes 10–12 in high yields and reaction times of
about 30 min while the subsequent TMS group removal using
TBAF (tetrabutylammonium fluoride) afforded desired
π-linkers 7c–9c (Scheme 2, Table 1, entries 10–12). A similar
synthesis of the linear π-linker 9c has already been reported
[40]. Moreover, unprotected terminal acetylenes 7c–9c showed
good stability with no signs of decomposition upon standing
over several months which facilitate their storage and use.
Supporting Information
Supporting Information File 1
Experimental procedures and characterization of
compounds.
[http://beilstein-journals.org/bjoc/content/supplementary/
1860-5397-5-11-S1.doc]
Supporting Information File 2
1H NMR spectra as well as GC/MS records for target
compounds 3–9.
[http://beilstein-journals.org/bjoc/content/supplementary/
1860-5397-5-11-S2.pdf]
Conclusion
It has been shown that extended, donor-substituted π-conjug- Acknowledgments
ated linkers can be easily prepared using commercially avail- This research was supported by the Ministry of Education,
able precursors under either conventional or modern synthetic Youth and Sport of the Czech Republic (MSM 0021627501 and
conditions. The reaction procedures reported here refer to the LA09041).
optimized procedures for each class of derivatives. Overall 12
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